The present invention relates to an apparatus and method for detecting a malfunction in a gas concentration detection system, for instance, in an oxygen concentration detection system for an internal combustion engine.
In an engine mounted on a vehicle, a linear air-fuel ratio sensor (an oxygen responsive limit-current type A/F sensor) is used for detecting the air-fuel ratio of air-fuel mixture gas supplied to the engine linearly over a wide air-fuel ratio zone. It is necessary to maintain temperature of the sensor to a predetermined activation temperature in order to detect the air-fuel ratio (concentration of oxygen in exhaust gas) with high accuracy. For this purpose, the sensor is generally provided with a heater and the on-state of the heater (electric power supply to heater) is controlled by a duty ratio control signal.
According to Japanese Patent Application Laid-Open No. 5-195843 (U.S. Pat. No. 5,327,780), a voltage applied to the heater of the A/F sensor is measured during both the on-state and off-state of the heater so that a trouble (malfunction) signal is generated when the difference between the measured voltages lies outside of a predetermined range. In this apparatus, however, since it is constructed mainly to discriminate the presence or absence of trouble on the basis of voltage values (power source voltages) applied to the heater, it is insufficient to specify the trouble part in the heater control system in a trouble mode. For an automobile in a repair shop or the like after occurrence of trouble, in order to investigate the cause of the trouble and which part is to be repaired, various checks such as a check of a control circuit, a harness check (including checks of lead wires and connectors) and the like are necessitated.
Further, according to Japanese Patent Application Laid-Open Nd. Sho 53-116896, a voltage applied to the A/F sensor is variably controlled in accordance with sensor current. In this apparatus, there will occur another malfunction. That is, sensor current exceeding a limit-current zone flows excessively when a positive or negative excessive voltage is applied to the sensor for some reason, thereby deteriorating the sensor operation.